This invention was made in part under National Science Foundation Grant No. DMR-8818558. The United States Government has certain rights to this invention.
The evolving demands for electronic packaging of very large scale integrated circuits require increasingly severe specifications of high integration, speed, and reliability. Accordingly, new materials with low dielectric constants, coefficients of thermal expansion compatible with the semiconductor component, and high thermal conductivity have been sought in order to accommodate to the increasing switching speeds of transistors. By lowering the dielectric constant of the substrate material, the thickness of the insulating layers can be reduced without loss of impedance.
One approach taken by the art to develop such new materials has involved the use of low dielectric organic polymers such as polyimide for the insulating layers. Polyimide exhibits a high glass transition temperature with relatively good thermal stability and mechanical properties. A serious drawback of polyimide, however, resides in its coefficient of thermal expansion being an order of magnitude higher than that of the semiconductor material. Additionally, organic polymers generally lack the thermal and mechanical stability, particularly in radiation environments, intrinsic to ceramic materials. For example, efforts to replace polyimide with various fluoropolymers have not met with much success. Despite their low permittivity, their strength and modulus are prohibitively low, thereby resulting in creep under load and when subjected to environmental thermal cycling.
Recent research has been directed to the fabrication of composite bodies consisting of a combination of a ceramic material and an organic phase. The objective of such research has been to produce articles wherein the desirable properties of the organic phase would be joined with the desired properties of the ceramic material. Illustrations of that research are disclosed in U. S. Pat. No. 4,239,519 (Beall et al.), U. S. Pat. No. 4,454,237 (Hoda et al.) and U. S. Pat. No. 4,455,382 (Wu).
U. S. Pat. No. 4,239,519 describes the preparation of organic-inorganic composite bodies containing crystals of a cation exchanged lithium and/or sodium water-swelling mica selected from the group of fluorhectorite, hydroxyl hectorite, boron fluorphlogopite, hydroxyl boron phlogopite, and solid solutions among those and between those and other structurally-compatible species selected from the group of talc, fluortalc, polylithionite, fluorpolylithionite, phlogopite, and fluorphlogopite. In the preferred inventive embodiment the starting water-swelling mica was prepared by forming a glass-ceramic body through the heat treatment of a precursor glass body; that glass body generally consisting essentially, in weight percent, of 0-12% Li.sub.2 O, 0-10% Na.sub.2 O, 0.5-14% Li.sub.2 O+Na.sub.2 O, 0-30% B.sub.2 O.sub.3, 10-38% MgO, 0-10% Al.sub.2 O.sub.3, 35-70% SiO.sub.2, 0-14% F, 0-15% OH, 4-15% F+OH, and, optionally, up to 30% total of oxides in the indicated proportions selected from the group of 0-10% CaO, 0-10% CdO, 0-10% CoO, 0-10% CuO, 0-20% FeO, 0-20% Fe.sub.2 O.sub.3, 0-10% MnO, 0-10% NiO, 0-10% P.sub.2 O.sub.5, 0-30% PbO, 0-20% SrO, 0-10% TiO.sub.2, 0-25% ZnO, and 0-10% ZrO.sub.2. The preferred base compositions consisted essentially of 0.5-12% Li.sub.2 O, 0-10% Na.sub.2 O, 0.5-14% Li.sub.2 O+Na.sub.2 O, 14-38% MgO, 0-15% B.sub. 2 O.sub.3, 0-10% Al.sub.2 O.sub.3, 35-70% SiO.sub.2, and 5-15% F. The glass-ceramic bodies prepared from the above composition intervals contained crystals selected from the group of fluorhectorite, boron fluorphlogopite, and solid solutions between those and between those and other structurally-compatible species selected from the group of talc, fluortalc, polylithionite, fluorpolylithionite, phlogopite, and fluorphlogopite. The above-described glass-ceramic body was contacted with a polar liquid, desirably water, to cause swelling and disintegration of the body accompanied with the formation of a gel. The solid-liquid ratio of the gel was adjusted to a desired fluidity depending upon the geometry of the ultimate article to be formed. The desired article was formed and then contacted with a source of large cations (most frequently a water solution thereof) to effect an ion exchange reaction between those cations and the Li.sup.+ and/or Na+ ions from the interlayer of the mica crystals with accompanying flocculation of the gel. Finally, the ion exchanged article was dried. The preferred large cations were selected from the group of Ag.sup.+, Ba.sup.+2, Ca.sup.+2 , Cu.sup.+2, Cs.sup.+, K.sup.+, NH.sub.4.sup.+, Pb.sup.+, Rb.sup.+, Sr.sup.+2, and certain organic polycations, specifically noting aniline hydrochloride and quaternary ammonium compounds. Hence, with the organic polycations as the substituting cations, the Li.sup.+ and/or Na.sup.+ ions present in the interlayer of the mica crystals are replaced with the organic polycations.
As is explained in U.S. Pat. No. 4,239,519, the crystals developed exhibit a morphology of a continuum of flakes, rectangular-like strips, and interwoven ribbons in parallel or sub-parallel zones or sheaths, with said flakes being irregularly shaped with diameters between about 0.5-10 microns and cross sections of less than 100.ANG., and said strips and ribbons being about 0.5-10 microns long, about 500-5000.ANG. wide, and less than about 100.ANG. thick. That morphology results in crystals demonstrating very high aspect ratios, higher than naturally-occurring micas, and large surface areas, both of those features serving to recommend their utility for reinforcing various matrices.
U. S. Pat. No. 4,454,237 discloses the preparation of an organic-inorganic composite body demonstrating hydrophobic behavior. The lithium and/or sodium waterswelling micas described in U. S. Pat. No. 4,239,519 comprised the starting materials, and the process utilized in preparing the body also followed that reported in that patent. The ion exchange reaction involved organic polycations selected from the group of aminosilanes and organic chrome complexes. As was observed in Hoda et al., only an amount of aminosilane and/or organic chrome complex sufficient to replace essentially all of the Li.sup.+ and/or Na.sup.+ ions in the interlayer of the mica will be employed.
U. S. Pat. No. 4,455,382 likewise discloses the preparation of an organic-inorganic composite body wherein the lithium and/or sodium water-swelling micas recorded in U. S. Pat. No. 4,239,519 comprise the starting materials and the process employed in preparing the body also followed that described in that patent. The ion exchange reaction involved organic polycations selected from the group of (a) a primary amine solubilized with acid, (b) a secondary amine solubilized with acid, (c) a tertiary amine solubilized with acid, (d) a quaternary ammonium acid salt, (e) a quaternary phosphonium acid salt, and (f) a ternary sulfonium acid salt. Again, only an amount sufficient to replace essentially all of the Li.sup.+ and/or Na.sup.+ ions in the interlayer of the mica will be utilized.
As can be appreciated, the concentration of the organic component is limited to that resulting from the replacement of Li.sup.+ and/or Na.sup.+ ions from the interlayer of the mica. Moreover, the operable organic species are restricted to organic cations.